# Semaphore

public class **Semaphore** extends [Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html)  
implements [Serializable](https://developers.google.com/j2objc/javadoc/jre/reference/java/io/Serializable.html) A counting semaphore. Conceptually, a semaphore maintains a set of
permits. Each `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquire()` blocks if necessary until a permit is
available, and then takes it. Each `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release()` adds a permit,
potentially releasing a blocking acquirer.
However, no actual permit objects are used; the `Semaphore` just
keeps a count of the number available and acts accordingly.

Semaphores are often used to restrict the number of threads than can
access some (physical or logical) resource. For example, here is
a class that uses a semaphore to control access to a pool of items:

     class Pool {
       private static final int MAX_AVAILABLE = 100;
       private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);

       public Object getItem() throws InterruptedException {
         available.acquire();
         return getNextAvailableItem();
       }

       public void putItem(Object x) {
         if (markAsUnused(x))
           available.release();
       }

       // Not a particularly efficient data structure; just for demo

       protected Object[] items = ... whatever kinds of items being managed
       protected boolean[] used = new boolean[MAX_AVAILABLE];

       protected synchronized Object getNextAvailableItem() {
         for (int i = 0; i < MAX_AVAILABLE; ++i) {
           if (!used[i]) {
              used[i] = true;
              return items[i];
           }
         }
         return null; // not reached
       }

       protected synchronized boolean markAsUnused(Object item) {
         for (int i = 0; i < MAX_AVAILABLE; ++i) {
           if (item == items[i]) {
              if (used[i]) {
                used[i] = false;
                return true;
              } else
                return false;
           }
         }
         return false;
       }
     }

Before obtaining an item each thread must acquire a permit from
the semaphore, guaranteeing that an item is available for use. When
the thread has finished with the item it is returned back to the
pool and a permit is returned to the semaphore, allowing another
thread to acquire that item. Note that no synchronization lock is
held when `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquire()` is called as that would prevent an item
from being returned to the pool. The semaphore encapsulates the
synchronization needed to restrict access to the pool, separately
from any synchronization needed to maintain the consistency of the
pool itself.

A semaphore initialized to one, and which is used such that it
only has at most one permit available, can serve as a mutual
exclusion lock. This is more commonly known as a *binary
semaphore* , because it only has two states: one permit
available, or zero permits available. When used in this way, the
binary semaphore has the property (unlike many `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/locks/Lock.html`
implementations), that the "lock" can be released by a
thread other than the owner (as semaphores have no notion of
ownership). This can be useful in some specialized contexts, such
as deadlock recovery.

The constructor for this class optionally accepts a
*fairness* parameter. When set false, this class makes no
guarantees about the order in which threads acquire permits. In
particular, *barging* is permitted, that is, a thread
invoking `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquire()` can be allocated a permit ahead of a
thread that has been waiting - logically the new thread places itself at
the head of the queue of waiting threads. When fairness is set true, the
semaphore guarantees that threads invoking any of the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquire()` methods are selected to obtain permits in the order in
which their invocation of those methods was processed
(first-in-first-out; FIFO). Note that FIFO ordering necessarily
applies to specific internal points of execution within these
methods. So, it is possible for one thread to invoke
`acquire` before another, but reach the ordering point after
the other, and similarly upon return from the method.
Also note that the untimed `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#tryAcquire()` methods do not
honor the fairness setting, but will take any permits that are
available.

Generally, semaphores used to control resource access should be
initialized as fair, to ensure that no thread is starved out from
accessing a resource. When using semaphores for other kinds of
synchronization control, the throughput advantages of non-fair
ordering often outweigh fairness considerations.

This class also provides convenience methods to `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquire(int)` and `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release(int)` multiple
permits at a time. These methods are generally more efficient and
effective than loops. However, they do not establish any preference
order. For example, if thread A invokes `s.acquire(3`) and
thread B invokes `s.acquire(2)`, and two permits become
available, then there is no guarantee that thread B will obtain
them unless its acquire came first and Semaphore `s` is in
fair mode.

Memory consistency effects: Actions in a thread prior to calling
a "release" method such as `release()`
[*happen-before*](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/package-summary#MemoryVisibility)
actions following a successful "acquire" method such as `acquire()`
in another thread.

### Public Constructor Summary

|---|---|
|   | [Semaphore](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#Semaphore(int))(int permits) Creates a `Semaphore` with the given number of permits and nonfair fairness setting. |
|   | [Semaphore](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#Semaphore(int,%20boolean))(int permits, boolean fair) Creates a `Semaphore` with the given number of permits and the given fairness setting. |

### Public Method Summary

|---|---|
| void | [acquire](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquire())() Acquires a permit from this semaphore, blocking until one is available, or the thread is {@linkplain Thread#interrupt interrupted}. |
| void | [acquire](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquire(int))(int permits) Acquires the given number of permits from this semaphore, blocking until all are available, or the thread is {@linkplain Thread#interrupt interrupted}. |
| void | [acquireUninterruptibly](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquireUninterruptibly(int))(int permits) Acquires the given number of permits from this semaphore, blocking until all are available. |
| void | [acquireUninterruptibly](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquireUninterruptibly())() Acquires a permit from this semaphore, blocking until one is available. |
| int | [availablePermits](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#availablePermits())() Returns the current number of permits available in this semaphore. |
| int | [drainPermits](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#drainPermits())() Acquires and returns all permits that are immediately available. |
| final int | [getQueueLength](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#getQueueLength())() Returns an estimate of the number of threads waiting to acquire. |
| final boolean | [hasQueuedThreads](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#hasQueuedThreads())() Queries whether any threads are waiting to acquire. |
| boolean | [isFair](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#isFair())() Returns `true` if this semaphore has fairness set true. |
| void | [release](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release(int))(int permits) Releases the given number of permits, returning them to the semaphore. |
| void | [release](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release())() Releases a permit, returning it to the semaphore. |
| [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html) | [toString](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#toString())() Returns a string identifying this semaphore, as well as its state. |
| boolean | [tryAcquire](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#tryAcquire(long,%20java.util.concurrent.TimeUnit))(long timeout, [TimeUnit](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeUnit.html) unit) Acquires a permit from this semaphore, if one becomes available within the given waiting time and the current thread has not been {@linkplain Thread#interrupt interrupted}. |
| boolean | [tryAcquire](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#tryAcquire(int,%20long,%20java.util.concurrent.TimeUnit))(int permits, long timeout, [TimeUnit](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeUnit.html) unit) Acquires the given number of permits from this semaphore, if all become available within the given waiting time and the current thread has not been {@linkplain Thread#interrupt interrupted}. |
| boolean | [tryAcquire](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#tryAcquire())() Acquires a permit from this semaphore, only if one is available at the time of invocation. |
| boolean | [tryAcquire](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#tryAcquire(int))(int permits) Acquires the given number of permits from this semaphore, only if all are available at the time of invocation. |

### Protected Method Summary

|---|---|
| [Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<Thread\> | [getQueuedThreads](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#getQueuedThreads())() Returns a collection containing threads that may be waiting to acquire. |
| void | [reducePermits](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#reducePermits(int))(int reduction) Shrinks the number of available permits by the indicated reduction. |

### Inherited Method Summary

From class [java.lang.Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html)

|---|---|
| [Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) | [clone](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#clone())() Creates and returns a copy of this `Object`. |
| boolean | [equals](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#equals(java.lang.Object))([Object](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html) obj) Compares this instance with the specified object and indicates if they are equal. |
| void | [finalize](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#finalize())() Invoked when the garbage collector has detected that this instance is no longer reachable. |
| final [Class](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Class.html)\<?\> | [getClass](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#getClass())() Returns the unique instance of `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Class.html` that represents this object's class. |
| int | [hashCode](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#hashCode())() Returns an integer hash code for this object. |
| final void | [notify](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notify())() Causes a thread which is waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up. |
| final void | [notifyAll](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#notifyAll())() Causes all threads which are waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up. |
| [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html) | [toString](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#toString())() Returns a string containing a concise, human-readable description of this object. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait(long,%20int))(long timeout, int nanos) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait(long))(long timeout) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires. |
| final void | [wait](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/Object.html#wait())() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object. |

## Public Constructors

#### public
**Semaphore**
(int permits)

Creates a `Semaphore` with the given number of
permits and nonfair fairness setting.

##### Parameters

| permits | the initial number of permits available. This value may be negative, in which case releases must occur before any acquires will be granted. |
|---|---|

#### public
**Semaphore**
(int permits, boolean fair)

Creates a `Semaphore` with the given number of
permits and the given fairness setting.

##### Parameters

| permits | the initial number of permits available. This value may be negative, in which case releases must occur before any acquires will be granted. |
| fair | `true` if this semaphore will guarantee first-in first-out granting of permits under contention, else `false` |
|---|---|

## Public Methods

#### public void
**acquire**
()

Acquires a permit from this semaphore, blocking until one is
available, or the thread is {@linkplain Thread#interrupt interrupted}.

Acquires a permit, if one is available and returns immediately,
reducing the number of available permits by one.

If no permit is available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
one of two things happens:

- Some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release()` method for this semaphore and the current thread is next to be assigned a permit; or
- Some other thread {@linkplain Thread#interrupt interrupts} the current thread.

If the current thread:

- has its interrupted status set on entry to this method; or
- is {@linkplain Thread#interrupt interrupted} while waiting for a permit,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` is thrown and the current thread's interrupted status is cleared.

<br />

##### Throws

| [InterruptedException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread is interrupted |
|---|---|

#### public void
**acquire**
(int permits)

Acquires the given number of permits from this semaphore,
blocking until all are available,
or the thread is {@linkplain Thread#interrupt interrupted}.

Acquires the given number of permits, if they are available,
and returns immediately, reducing the number of available permits
by the given amount. This method has the same effect as the
loop `for (int i = 0; i < permits; ++i) acquire();` except
that it atomically acquires the permits all at once:

If insufficient permits are available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
one of two things happens:

- Some other thread invokes one of the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release()` methods for this semaphore and the current thread is next to be assigned permits and the number of available permits satisfies this request; or
- Some other thread {@linkplain Thread#interrupt interrupts} the current thread.

If the current thread:

- has its interrupted status set on entry to this method; or
- is {@linkplain Thread#interrupt interrupted} while waiting for a permit,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` is thrown and the current thread's interrupted status is cleared. Any permits that were to be assigned to this thread are instead assigned to other threads trying to acquire permits, as if permits had been made available by a call to `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release()`.

<br />

##### Parameters

| permits | the number of permits to acquire |
|---|---|

##### Throws

| [InterruptedException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread is interrupted |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if `permits` is negative |
|---|---|

#### public void
**acquireUninterruptibly**
(int permits)

Acquires the given number of permits from this semaphore,
blocking until all are available.

Acquires the given number of permits, if they are available,
and returns immediately, reducing the number of available permits
by the given amount. This method has the same effect as the
loop `for (int i = 0; i < permits; ++i) acquireUninterruptibly();`
except that it atomically acquires the permits all at once:

If insufficient permits are available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
some other thread invokes one of the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release()`
methods for this semaphore and the current thread is next to be assigned
permits and the number of available permits satisfies this request.

If the current thread is {@linkplain Thread#interrupt interrupted}
while waiting for permits then it will continue to wait and its
position in the queue is not affected. When the thread does return
from this method its interrupt status will be set.

##### Parameters

| permits | the number of permits to acquire |
|---|---|

##### Throws

| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if `permits` is negative |
|---|---|

#### public void
**acquireUninterruptibly**
()

Acquires a permit from this semaphore, blocking until one is
available.

Acquires a permit, if one is available and returns immediately,
reducing the number of available permits by one.

If no permit is available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release()` method for this
semaphore and the current thread is next to be assigned a permit.

If the current thread is {@linkplain Thread#interrupt interrupted}
while waiting for a permit then it will continue to wait, but the
time at which the thread is assigned a permit may change compared to
the time it would have received the permit had no interruption
occurred. When the thread does return from this method its interrupt
status will be set.

#### public int
**availablePermits**
()

Returns the current number of permits available in this semaphore.

This method is typically used for debugging and testing purposes.

##### Returns

- the number of permits available in this semaphore

#### public int
**drainPermits**
()

Acquires and returns all permits that are immediately available.

##### Returns

- the number of permits acquired

#### public final int
**getQueueLength**
()

Returns an estimate of the number of threads waiting to acquire.
The value is only an estimate because the number of threads may
change dynamically while this method traverses internal data
structures. This method is designed for use in monitoring
system state, not for synchronization control.

##### Returns

- the estimated number of threads waiting for this lock

#### public final boolean
**hasQueuedThreads**
()

Queries whether any threads are waiting to acquire. Note that
because cancellations may occur at any time, a `true`
return does not guarantee that any other thread will ever
acquire. This method is designed primarily for use in
monitoring of the system state.

##### Returns

- `true` if there may be other threads waiting to acquire the lock

#### public boolean
**isFair**
()

Returns `true` if this semaphore has fairness set true.

##### Returns

- `true` if this semaphore has fairness set true

#### public void
**release**
(int permits)

Releases the given number of permits, returning them to the semaphore.

Releases the given number of permits, increasing the number of
available permits by that amount.
If any threads are trying to acquire permits, then one thread
is selected and given the permits that were just released.
If the number of available permits satisfies that thread's request
then that thread is (re)enabled for thread scheduling purposes;
otherwise the thread will wait until sufficient permits are available.
If there are still permits available
after this thread's request has been satisfied, then those permits
are assigned in turn to other threads trying to acquire permits.

There is no requirement that a thread that releases a permit must
have acquired that permit by calling `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquire()`.
Correct usage of a semaphore is established by programming convention
in the application.

##### Parameters

| permits | the number of permits to release |
|---|---|

##### Throws

| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if `permits` is negative |
|---|---|

#### public void
**release**
()

Releases a permit, returning it to the semaphore.

Releases a permit, increasing the number of available permits by
one. If any threads are trying to acquire a permit, then one is
selected and given the permit that was just released. That thread
is (re)enabled for thread scheduling purposes.

There is no requirement that a thread that releases a permit must
have acquired that permit by calling `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#acquire()`.
Correct usage of a semaphore is established by programming convention
in the application.

#### public [String](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/String.html)
**toString**
()

Returns a string identifying this semaphore, as well as its state.
The state, in brackets, includes the String `"Permits ="`
followed by the number of permits.

##### Returns

- a string identifying this semaphore, as well as its state

#### public boolean
**tryAcquire**
(long timeout, [TimeUnit](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeUnit.html) unit)

Acquires a permit from this semaphore, if one becomes available
within the given waiting time and the current thread has not
been {@linkplain Thread#interrupt interrupted}.

Acquires a permit, if one is available and returns immediately,
with the value `true`,
reducing the number of available permits by one.

If no permit is available then the current thread becomes
disabled for thread scheduling purposes and lies dormant until
one of three things happens:

- Some other thread invokes the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release()` method for this semaphore and the current thread is next to be assigned a permit; or
- Some other thread {@linkplain Thread#interrupt interrupts} the current thread; or
- The specified waiting time elapses.

If a permit is acquired then the value `true` is returned.

If the current thread:

- has its interrupted status set on entry to this method; or
- is {@linkplain Thread#interrupt interrupted} while waiting to acquire a permit,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` is thrown and the current thread's interrupted status is cleared.

If the specified waiting time elapses then the value `false`
is returned. If the time is less than or equal to zero, the method
will not wait at all.

##### Parameters

| timeout | the maximum time to wait for a permit |
| unit | the time unit of the `timeout` argument |
|---|---|

##### Returns

- `true` if a permit was acquired and `false` if the waiting time elapsed before a permit was acquired

##### Throws

| [InterruptedException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread is interrupted |
|---|---|

#### public boolean
**tryAcquire**
(int permits, long timeout, [TimeUnit](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/TimeUnit.html) unit)

Acquires the given number of permits from this semaphore, if all
become available within the given waiting time and the current
thread has not been {@linkplain Thread#interrupt interrupted}.

Acquires the given number of permits, if they are available and
returns immediately, with the value `true`,
reducing the number of available permits by the given amount.

If insufficient permits are available then
the current thread becomes disabled for thread scheduling
purposes and lies dormant until one of three things happens:

- Some other thread invokes one of the `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release()` methods for this semaphore and the current thread is next to be assigned permits and the number of available permits satisfies this request; or
- Some other thread {@linkplain Thread#interrupt interrupts} the current thread; or
- The specified waiting time elapses.

If the permits are acquired then the value `true` is returned.

If the current thread:

- has its interrupted status set on entry to this method; or
- is {@linkplain Thread#interrupt interrupted} while waiting to acquire the permits,

then `https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html` is thrown and the current thread's interrupted status is cleared. Any permits that were to be assigned to this thread, are instead assigned to other threads trying to acquire permits, as if the permits had been made available by a call to `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release()`.

If the specified waiting time elapses then the value `false`
is returned. If the time is less than or equal to zero, the method
will not wait at all. Any permits that were to be assigned to this
thread, are instead assigned to other threads trying to acquire
permits, as if the permits had been made available by a call to
`https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#release()`.

##### Parameters

| permits | the number of permits to acquire |
| timeout | the maximum time to wait for the permits |
| unit | the time unit of the `timeout` argument |
|---|---|

##### Returns

- `true` if all permits were acquired and `false` if the waiting time elapsed before all permits were acquired

##### Throws

| [InterruptedException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/InterruptedException.html) | if the current thread is interrupted |
| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if `permits` is negative |
|---|---|

#### public boolean
**tryAcquire**
()

Acquires a permit from this semaphore, only if one is available at the
time of invocation.

Acquires a permit, if one is available and returns immediately,
with the value `true`,
reducing the number of available permits by one.

If no permit is available then this method will return
immediately with the value `false`.

Even when this semaphore has been set to use a
fair ordering policy, a call to `tryAcquire()` *will*
immediately acquire a permit if one is available, whether or not
other threads are currently waiting.
This "barging" behavior can be useful in certain
circumstances, even though it breaks fairness. If you want to honor
the fairness setting, then use
`https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#tryAcquire(long,%20java.util.concurrent.TimeUnit)`
which is almost equivalent (it also detects interruption).

##### Returns

- `true` if a permit was acquired and `false` otherwise

#### public boolean
**tryAcquire**
(int permits)

Acquires the given number of permits from this semaphore, only
if all are available at the time of invocation.

Acquires the given number of permits, if they are available, and
returns immediately, with the value `true`,
reducing the number of available permits by the given amount.

If insufficient permits are available then this method will return
immediately with the value `false` and the number of available
permits is unchanged.

Even when this semaphore has been set to use a fair ordering
policy, a call to `tryAcquire` *will*
immediately acquire a permit if one is available, whether or
not other threads are currently waiting. This
"barging" behavior can be useful in certain
circumstances, even though it breaks fairness. If you want to
honor the fairness setting, then use `https://developers.google.com/j2objc/javadoc/jre/reference/java/util/concurrent/Semaphore.html#tryAcquire(int,%20long,%20java.util.concurrent.TimeUnit)`
which is almost equivalent (it also detects interruption).

##### Parameters

| permits | the number of permits to acquire |
|---|---|

##### Returns

- `true` if the permits were acquired and `false` otherwise

##### Throws

| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if `permits` is negative |
|---|---|

## Protected Methods

#### protected [Collection](https://developers.google.com/j2objc/javadoc/jre/reference/java/util/Collection.html)\<Thread\>
**getQueuedThreads**
()

Returns a collection containing threads that may be waiting to acquire.
Because the actual set of threads may change dynamically while
constructing this result, the returned collection is only a best-effort
estimate. The elements of the returned collection are in no particular
order. This method is designed to facilitate construction of
subclasses that provide more extensive monitoring facilities.

##### Returns

- the collection of threads

#### protected void
**reducePermits**
(int reduction)

Shrinks the number of available permits by the indicated
reduction. This method can be useful in subclasses that use
semaphores to track resources that become unavailable. This
method differs from `acquire` in that it does not block
waiting for permits to become available.

##### Parameters

| reduction | the number of permits to remove |
|---|---|

##### Throws

| [IllegalArgumentException](https://developers.google.com/j2objc/javadoc/jre/reference/java/lang/IllegalArgumentException.html) | if `reduction` is negative |
|---|---|